1Field of the Invention
The present invention relates to an apparatus for controlling a thermal head, such as may be used in a facsimile or thermal printer.
2. Description of the Related Art
In general, the thermal head performs the printing by selectively energizing a plurality of heating elements to color a heat-sensitive sheet of paper. Each of the heating elements may include a heating resistance element and common and IC leads which are used as electrodes connected to the heating resistance element. The common lead is used to apply a bias voltage to the heating resistance element while the IC lead is connected to a driver IC which is turned on or off for each pixel in response to a print signal. As the bias voltage is being applied to the heating resistance element, the driver IC is selectively driven by the print signal to energize the heating resistance element.
When the bias voltage is being applied to the thermal head under a condition of high temperature/humidity, there may be produced a break-down known as "electric erosion" by which the electrodes and heating resistance elements are eroded by an electrochemical reaction.
Such an electric erosion will be described with reference to FIG. 4. A thermal head shown in FIG. 4 comprises a substrate 1, a resistor 2 formed on the substrate 1 and common and IC leads 3, 4 similarly formed on the substrate 1. The resistor and parts of the leads are coated with an overcoat glass 5. The IC lead 4 is connected to a driver IC 6. A heat-sensitive sheet of paper 7 is moved by a platen roller 8 while contacting the overcoat glass 5. The resistor 2 may be made of RuO.sub.2 while the common and IC leads 3, 4 may be formed of Au.
The mechanism of electric erosion is believed to be as follows. The heat-sensitive sheet 7 may contain Cl ions or Na ions. The Cl ions may pass through the mesh structure of the protective film or overcoat glass 5 to the resistor 2 by the aid of H.sub.2 O. The Cl ions then reach the gold patterns (leads 3 and 4) which are electrically conductive. The mixture of Cl+H.sub.2 creates an electrolytic reaction at the electrode parts of the gold patterns so that the gold will migrate to the heat-sensitive sheet 7. Such a reaction proceeds with increasing speed until the gold patterns are broken. At the same time, a pin hole or holes may be formed in the protective film. The electrochemical reaction is believed to include the following reactions produced in parallel with each other: EQU Cl.sup.- +Au+H.sub.2 O+e.sup.- .fwdarw.HAuCl.sub.4 4H.sub.2 EQU Na.sup.+ +S+Au+H.sub.2 O+e.sup.- .fwdarw.Na.sub.3 Au(SO.sub.3).sub.2 .chi.H.sub.2 O
It is believed that the influence of Cl on the protective film is different from that of Na. There is an opinion that on application of the voltage, the Cl ions form a fine hole or holes on the protective film in the positive direction or in the direction toward the conductor. On the other hand, the Na ions move relatively freely in the protective film (glass layer) and tend to be attracted in the negative direction or toward the platen roller 8 or heat-sensitive sheet 7. Therefore, the Na ions erode the surface of the protective film in the form of NaOH or the like.
In order to avoid such an electric erosion, it may be preferred that the bias voltage is not applied to the thermal head on non-printing. Depending on the specification of a device using such a thermal head, a common line for supplying a motor both with power and bias may be used in the thermal head. In such a case, the bias voltage may accidentally be applied to the thermal head on non-printing to create the electric erosion. The prior art thus has difficulty in completely preventing the electric erosion.